Research brief: Hybrid Cyclones in the Australian Region

In May and September 2016, two extratropical cyclones developed explosively over the Great Australian Bight. The May cyclone affected the Eyre Peninsula and Adelaide region with storm-force winds and a damaging storm surge along the coast. The strong winds and record high sea levels caused coastal erosion and damaged power lines, buildings and other infrastructure.

The September cyclone and its associated cold front affected much the same region with destructive winds, intense rainfall and severe thunderstorms, including hail and at least seven tornadoes. The winds severely damaged the South Australian power network resulting in a state-wide power outage.

Both cyclones fall into a class known as hybrid cyclones.

Hybrid cyclones are relatively cold in the upper troposphere like a typical extratropical cyclone and warm in the lower troposphere like a tropical cyclone. Two papers by Centre of Excellence researchers are the first to systematically investigate and document this type of cyclone in the Australian region.

Among the most important findings are that:

most hybrid cyclones develop from May to September.

hybrid cyclones most frequently form over the Tasman Sea and the Great Australian Bight where they account for 50% of all extratropical cyclones.

the strongest precipitation, which is locally extreme in 91% of all cases, falls in the warm sector and along the associated warm front on the poleward side of the cyclone.

the area affected by extreme wind gusts is significantly larger in hybrid cyclones than other extratropical cyclones.

using statistical clustering and a theoretical approach known as piecewise potential vorticity inversion, we discovered the processes controlling the motion of hybrid cyclones in the region; they turn out to be much the same as the processes that control the motion of tropical cyclones.